1. Field of the Invention
The present invention relates to a diagnostic apparatus and a diagnostic method for a fuel vapor treating apparatus that introduces fuel vapor produced in a fuel tank into an intake system and then treats the fuel vapor.
2. Description of the Related Art
A diagnostic apparatus for a fuel vapor treating apparatus is disclosed in, for example, Japanese Patent Application Laid-Open No. HEI 6-235355. This diagnostic apparatus introduces negative pressure from an intake system into a purge system provided for introducing fuel vapor produced in a fuel tank into the intake system, and detects the value of a negative pressure in the purge system that converges after the introduction of negative pressure thereto. Then, the diagnostic apparatus compares the detected negative pressure value with a leak criterion to perform failure diagnosis.
This diagnostic apparatus performs diagnosis by detecting transition of pressure introduced into the purge system. As the space volume defined in the purge system is increased, the time needed for convergence of pressure value becomes longer. If the diagnostic time is long, external disturbances occurring during the diagnosis, such as an air temperature change, an air pressure change, a fuel temperature change, a change in the amount of fuel evaporation, a change in the amount of fuel consumption, and the like, may cause complex effects and thereby change the pressure in the purge system during the diagnosis. Therefore, the conventional diagnostic apparatus has a danger of making a false determination of a fault or failure.